Extensional faulting and landscape evolution
Understanding the interaction between earthquakes, geomorphic processes, and mountain landscape evolution is important from a variety of perspectives, including natural hazard management. One way to discover the physics behind mountain landscape evolution is to study natural experiments: case studies in which nature provides enough control that we can use them to test mathematical models. To help develop a better understanding of how earthquakes and erosion conspire to shape landscapes in areas of active tectonic extension, we have focused on two case studies: the Apennines Mountains of central Italy, and the Wasatch Fault System in the western United States. In both of these regions, stretching of the earth’s crust has created a network of mountain blocks that are moving relative to one another at rates that have been well documented. One of our main aims has been to examine how rivers in this region have responded to different rates and patterns of rock motion, and use the resulting data to evaluate mathematical models for landscape evolution.
One element of this project looks at the development of facet slopes: steep, often planar and rocky hillslopes that develop along and above the trace of an extensional (a.k.a. normal) fault. These surfaces record progressive exhumation and erosion as rocks in the footwall block are lifted upward and outward, relative to the hangingwall block, by the accumulated action of earthquakes. One of the things we have learned is that the steepness and soil cover of facets reflects the competition between tectonic motion and geomorphic processes. An interesting implication of this finding is that all else being equal, faster-slipping faults produce steeper facets: an observation that can be used to identify differences in tectonic rates.
We have also developed a novel method for simulating facets and other steep, rocky slopes: a type of landform that has been tricky to model because it involves such a huge rangle of soil-motion speeds, from slow creeping to nearly instantaneous falling and bouncing of detached rocks. The method uses a technique called cellular automata simulation, and is described in three papers by Tucker et al. (2016, 2018, 2020).
Thick line shows longitudinal profile of a bedrock river channel on an active fault block whose motion accelerated during the Pleistocene period, resulting in steepening of the lower part of the channel; this is compared with computed profiles using both static and dynamic representations of channel width. (Lower left) Time sequence of simulated river profiles; stars indicate back-tilting and deflection of drainage in upper catchment due to fault-block rotation. (Figures from Attal, Tucker, Whittaker, Cowie, and Roberts, 2008 JGR-Earth Surface)
Tucker, G. E., Hobley, D.E.J., McCoy, S.W., and Struble, W.T. (2020) Modeling the shape and evolution of normal-fault facets. Journal of Geophysical Research: Earth Surface, 125, https://doi.org/10.1029/2019JF005305.
Tucker, G. E., McCoy, S.W., and Hobley, D.E.J. (2018) A lattice grain model of hillslope evolution. Earth Surface Dynamics, v. 6, p. 563-582, https://doi.org/10.5194/esurf-6-563-2018.
Tucker, G.E., Hobley, D.E.J., Hutton, E., Gasparini, N.M., Istanbulluoglu, E., Adams, J.M., and Nudurupati, S.S. (2016) CellLab-CTS 2015: Continuous-time stochastic cellular automaton modeling using Landlab. Geoscientific Model Development., v. 9, p. 823-839, https://doi.org/10.5194/gmd-9-823-2016.
Tucker, G. E., McCoy, S. W., Whittaker, A. C., Roberts, G. P., Lancaster, S. T., & Phillips, R. (2011). Geomorphic significance of postglacial bedrock scarps on normal‐fault footwalls. Journal of Geophysical Research: Earth Surface, 116(F1), https://doi.org/10.1029/2010JF001861.
Attal, M., Cowie, P. A., Whittaker, A. C., Hobley, D., Tucker, G. E., & Roberts, G. P. (2011). Testing fluvial erosion models using the transient response of bedrock rivers to tectonic forcing in the Apennines, Italy. Journal of Geophysical Research: Earth Surface, 116(F2), https://doi.org/10.1029/2010JF001875.
Attal, M., Tucker, G.E., Whittaker, A.C., Cowie, P.A., and Roberts, G.P. (2008) Modeling fluvial incision and transient landscape evolution: Influence of dynamic channel adjustment: Journal of Geophysical Research, v. 113, doi:10.1029/2007JF000893.
Cowie, P.A., Whittaker, A.C., Attal, M., Roberts, G., Tucker, G.E., and Ganas, A. (2008) New constraints on sediment-flux-dependent river incision: implications for extracting tectonic signals from river profiles. Geology, v. 36, no. 7, p. 535-538.
Whittaker, A.C., Attal, M., Cowie, P.A., Tucker, G.E., and Roberts, G.P. (2008) Decoding temporal and spatial patterns of fault uplift using transient river long profiles: Geomorphology, v. 100, no. 3-4, p. 506-526, doi:10.1016/j.geomorph.2008.01.018.
Whittaker, A.C., Cowie, P.A., Attal, M., Tucker, G.E., and Roberts, G.P. (2007) Bedrock channel adjustment to tectonic forcing: implications for predicting river incision rates: Geology, v. 35, no. 2, p. 103-106 (doi:10.1130/G23106A.1).
Whittaker, A.C., Cowie, P.A., Attal, M., Tucker, G.E., and Roberts, G.P. (2007) Contrasting transient and steady-state rivers crossing active normal faults: new field observations from the Central Apennines, Italy: Basin Research (doi:10.1111/j.1365-2117.2007.00337.x).
Cowie, P.A., Attal, M., Tucker, G.E., Whittaker, A.C., Naylor, M., Ganas, A., and Roberts, G.P. (2006) Investigating the surface process response of fault interaction and linkage using a numerical modeling approach: Basin Research, v. 18, p. 231-266.